Pyrolysis refers to the thermal decomposition of substances (feedstock) by heating in an oxygen deficient environment. Pyrolysis processes are generally divided into slow and fast pyrolysis according to the rate at which the feedstock is heated up. The traditional method of charcoal production is considered to be a slow pyrolysis process; it gives a liquid yield of typically around 30% (on dry mass basis). The fast pyrolysis of feedstock, on the other hand, gives a much higher liquid yield, widely reported to be above 60% (on dry mass basis). Therefore, the fast pyrolysis is advantageous over the slow pyrolysis when the targeted product from a carbonaceous feedstock is organic liquid
Some of the fast pyrolysis technologies have reached demonstration scale (e.g., fluidized bed, vacuum). However, these technologies may have substantive technical and economical constraints at the commercial scale. For example, fluidized bed pyrolysers require the use of very fine particles, typically below 2 mm in diameter, but the preparation of such fine feedstock particles usually incurs high costs. Another serious deficiency of fluidized beds is that a very large volume of inert gas is required in order to fluidize the inert solids and to carry the resulted char out of the reactor. Many problems are associated with the use of a large volume of inert gas. For example, the partial pressure of the organic vapour may become extremely low due to dilution by the inert gas, which makes it difficult to quench and collect the organic components. In addition, a high velocity gas flow leads to the formation of aerosols which are extremely difficult to collect. Furthermore, non-condensable pyrolysis gas generated in this process is also diluted by the large volume of inert gas, which makes the direct utilization of this gas more problematic (e.g. in a combustion facility for generating heat). The continuous heating up (before entering the fluidized bed) and subsequent cooling down (to condense the organic vapour as bio-oil) of the inert gas greatly reduces the overall process efficiency.